1. Field of Invention
This invention relates to the field of x-ray spectrometers, specifically to an instrument which is a combination energy dispersive and wavelength dispersive x-ray spectrometer and is capable of very quickly analyzing the elemental composition of an unknown sample by simultaneous application of wavelength dispersive and energy dispersive x-ray spectrometry.
2. Description of Prior Art
X-ray spectroscopy is performed on unknown samples to determine their elemental composition through characterization of x-rays emitted by the sample. X-ray spectroscopy can either be accomplished through energy dispersive methods or wavelength dispersive methods. An energy dispersive spectrometer (EDS) usually relies on pulse height analysis to separate the various x-rays into small energy bins. Pulses are produced when an x-ray impinges on a detector which emits a signal whose amplitude or integrated intensity is a function of the x-ray photon energy. A wavelength dispersive spectrometer (WDS) employs a diffracting element such as a crystal, synthetic multilayer or diffraction grating to separate the incident x-rays into their various wavelengths. The dispersing element is scanned through various angles corresponding to the x-ray wavelengths and the x-rays are diffracted into a detector which also usually moves to intercept these diffracted x-rays. Both systems have various strengths and weaknesses and are usually used separately.
Energy dispersive spectrometers have few or no moving parts and have been developed so that they are easy to use. Unfortunately, they also have poor energy resolution so they are unable to separate closely spaced peaks. For example, N has a spectral line at 0.392 KeV and Ti has a line at 0.452 KeV and they cannot be separated by most energy dispersive spectrometers. There are many other examples where one element in a matrix of another cannot be separated by energy dispersive spectroscopy. In addition, energy dispersive systems are not very efficient at detecting x-rays from the light elements, those with x-ray energies below 1 KeV. However, energy dispersive systems are so much simpler, easier to use and produce a spectrum in shorter time than wavelength dispersive systems that they are used far more than wavelength systems. Data from an EDS is usually displayed as a continuous spectrum, also termed parallel data collection, where peak heights grow as data is collected. Data collection can be terminated when sufficient information has been obtained.
Wavelength dispersive spectrometers typically have many moving parts and are very inefficient, thus data collection times for WDS systems are long. However, resolution is sufficiently good that almost all closely spaced peaks can be separated. Because wavelength systems are good at detecting and resolving low energy peaks, they are typically used for analysis of the light elements. Data is displayed as the crystal or other dispersing element slowly scans over the angles corresponding to the energies of interest. The scan rate is necessarily slow because the collection of x-rays is inefficient, resulting in long data collection times. The spectrum is slowly traced as the diffractor scans, so that it is displayed in a serial manner. The parallel data collection of the energy dispersive spectrometer would be preferred, but due to the slow scan rate, this has not been possible for a WDS before the present invention.